Feature Article

Maintaining Polymer Colour and Clarity when Using Radiation Sterilisation Technologies


Posted in Sterilization Services by Camilla Andersson on November 5, 2012

Gamma and E-beam sterilisation methods are fast and effective. To take full advantage of these attributes, however, manufacturers should research the newest families of copolyesters that retain the glass-like clarity, colour stability and in-use toughness desired by clinical staff. 


The traditional use of autoclave sterilisation for medical devices is starting to decrease. The autoclave raises the temperature of the device up to 132°C by applying pressurised steam at 30 psi or 2 bar. Using high temperatures and steam to kill micro-organisms can cause many commonly used polymers to warp and distort, rendering the devices ineffective. In addition, molecular breakdown of some polymers can occur through hydrolysis, making the devices susceptible to breakage.

 Figure 1: Photographs of moulded resins before and after exposure to 50 kGy of gamma radiation.

The high temperatures needed for autoclave sterilisation may melt some plastic products and blunt sharp instruments. Further, autoclaving is not the most time-effective sterilisation method and it requires a significant amount of space to allow sufficient circulation around the devices and to accommodate sterilisation temperatures. However, autoclave sterilisation does have the benefit of rarely causing significant colour shift in the polymeric materials used for packaging or devices.

To overcome the limitations of autoclave sterilisation, healthcare professionals (HCPs) increasingly are turning to radiation-based sterilisation methods, which bring different challenges.

The move to radiation sterilisation
Gamma and E-beam are becoming the preferred sterilisation methods. These techniques reduce cost by means of high-volume throughput, speed and the elimination of high-temperature processing.

 
 Figure 1: b* colour measurements of Eastman specialty plastics and competitive resins after exposure to 50 kGy of gamma radiation.

Gamma or E-beam techniques allow HCPs to sterilise the whole kit at once, as the radiation penetrates outer packaging and reaches the devices inside. This eliminates unpacking and repacking steps, reducing the possibility for human error and increasing sterility.

 
 Figure 3: b* colour measurements of Eastman specialty plastics and competitive resins after exposure to 50 kGy of e-beam sterilisation.

The major disadvantage of gamma sterilisation is the need for a cobalt-60 radioactive source, requiring special handling and additional costs. E-beam radiation does not require a radioactive source; however, the technique has limitations in penetrating high-density materials such as metals.

It is also important to understand how to minimise the impact of a sterilisation method on the optical properties of devices through proper material choice.

One of the most critical effects of sterilisation on a material is a change in colour. Polymers exposed to radiation often shift in colour to yellow. For HCPs, any discolouration or property degradation is a cause for concern, because colour-coding and clarity play a vital role in the functionality and reliability of medical devices.

Clarity for clinical staff
It’s important for medical devices to maintain their glass-like clarity and avoid colour shift during the sterilisation process, as HCPs often rely on colour coding for quick identification. During medical procedures, HCPs must be able to quickly, efficiently and correctly identify and select the correct device based on colour. This is often used to indicate the size, type and function of the device. Subsequent quick execution can help to minimise medical errors and save time.

Additionally, excellent clarity gives HCPs unobstructed views to enable them to more easily and quickly identify colour changes in the blood during perfusion, foreign substances and bubbles and clots. It also allows them to easily monitor fluid levels and ensure that medicine is being properly delivered to the patient. The identification of potential issues before they develop can prevent more serious health problems such as embolisms or insertion-site infections.

Clarity also affects patient confidence, a more emotive but no less important consideration. A yellow or hazed device gives the impression that it is old or has been previously used. It’s essential that patients have the utmost confidence in the procedure they are undergoing and the devices that are being used.

 Figure 4: Photographs of moulded resins before and after exposure to 50 kGy of e-beam radiation.

The effects of sterilisation on the colour and clarity of a particular material depend on the material’s inherent chemistry as well as the sterilisation method used. With radiation sterilisation, the type and dose of radiation, together with the length of time the sterilised parts are left to rest after radiation, affect the final colour.

Colour-shifting study
Eastman Chemical conducted a study to further understand how gamma and E-beam radiation affect various types of polymers. The study included medical-grade copolyesters, gamma-stable polycarbonate (PC), transparent acrylonitrile butadiene styrene (TABS), and acrylic (PMMA). To determine the initial colour and any changes, the CIE Lab* scale was used. (CIE Lab was established by the International Commission on Illumination as a standard method for comparing colour values.)

The test results indicated that gamma and E-beam radiation can affect the colour of various polymer materials differently. After being exposed to 50 kGy of gamma radiation and measured three days after initial exposure, each of the materials showed a positive shift to higher b* values, indicating yellowing. The medical-grade copolyester showed the least amount of initial yellowing, with a shift of only about 2 b* units. The acrylic, TABS and PC materials all shifted more than 20 b* units.

After 42 days, the yellow colour of each of the gamma-radiated test samples faded to varying degrees. The colour of the medical-grade copolyester sample faded to a final b* value within 0.2 units of the initial colour. The final colour for the TABS and acrylic samples faded to within approximately 6 and 9 b* units of the initial colour, respectively, while the PC sample faded the least, with a final colour shift of 17 b* units from the initial point.

After the materials were exposed to E-beam radiation, similar results were observed. The medical-grade copolyester had the lowest yellow shift of any of the materials with a measured b* shift of 4.3 units. The TABS and acrylic shifted approximately 11 and 8 b* units, respectively, and the PC shifted the most, with a change of 17 b* units. Again, the colour faded towards the initial colour over time, with the medical-grade copolyester sample coming to within 0.5 b* units. The TABS and acrylic samples’ final colour shift was slightly less than 3 b* units, while the final colour of the PC sample was 9 b* units higher than that of the original.

Overall, medical-grade copolyesters showed insignificant colour shifts and had the lowest overall colour shift after radiation sterilisation among all the polymers tested. After gamma and E-beam sterilisation, one of the newest copolyesters, Eastman Tritan copolyester, exhibited best-in-class colour retention among the transparent polymers available for medical device applications.

Resistance to in-use abuse
Use of copolyesters for manufacturing medical devices minimises the colour shift or hazing after gamma or E-beam radiation sterilisation, thereby maintaining optimal clarity and colour and enabling medical device manufacturers to take full advantage of the speed and convenience of this type of sterilisation.

The recently developed copolyesters are heat-resistant, tough and sterilisation–stable, making them suitable for gamma or E-beam radiation. The new materials are also able to withstand exposure to blood, lipids and chemical agents, including isopropyl alcohol, disinfectants and bonding solvents.

Copolyesters that offer clarity and stability of colour and properties after sterilisation are suitable for use in a range of applications such as dialyser housings, syringes, blood separation devices, wound-treatment applications, blood-collection tubes, respiratory devices and IV components. Copolyesters are engineered to meet the performance demands of today’s injection-moulded medical devices whilst providing toughness, chemical resistance, processability and transparency.

Hospital conditions require medical devices to meet emergency treatment needs such as tapping and accidental knocking. The inherent toughness of copolyesters means that they are more resistant to everyday clinical abuse. They maintain their key mechanical and physical properties following sterilisation, which makes copolyesters well-suited to meet the material durability requirements of a hospital environment.

Resistance to solvents like lipids and isopropanol allows these versatile materials to be tailored to the specific needs of both the OEM and end user. Additionally, copolyesters are bisphenol A-free and are generally considered to be biocompatible, nontoxic, environmentally friendly materials, making them a good fit with the medical industry’s evolving green strategies.

 

Theo Wubbels
is Market Development Manager EMEA, Medical Specialty Plastics
Fascinatio Boulevard 602-614,
2909 VA, Capelle aan den Ijssel, Netherlands
Email: twubbels@eastman.com
Mobile: +31 61 383 2642



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